A higher supergroup for string theory
A higher supergroup for string theory
John Huerta
http://math.ucr.edu/∼huerta
Mathematical Sciences Institute Australian National University
Adelaide 5 September 2011 The explanation involves ‘higher gauge theory’.
A higher supergroup for string theory Introduction
This research began as a puzzle. Explain this pattern:
I The only normed division algebras are R, C, H and O. They have dimensions k = 1, 2, 4 and 8.
I The classical superstring makes sense only in dimensions k + 2 = 3, 4, 6 and 10.
I The classical super-2-brane makes sense only in dimensions k + 3 = 4, 5, 7 and 11. A higher supergroup for string theory Introduction
This research began as a puzzle. Explain this pattern:
I The only normed division algebras are R, C, H and O. They have dimensions k = 1, 2, 4 and 8.
I The classical superstring makes sense only in dimensions k + 2 = 3, 4, 6 and 10.
I The classical super-2-brane makes sense only in dimensions k + 3 = 4, 5, 7 and 11.
The explanation involves ‘higher gauge theory’. since composition of paths then corresponds to multiplication:
g g0 • % • % •
while reversing the direction corresponds to taking the inverse:
g−1 ••y
A higher supergroup for string theory Introduction
Ordinary gauge theory describes how 0-dimensional particles transform as we move them along 1-dimensional paths. It is natural to assign a Lie group element to each path:
g • % • while reversing the direction corresponds to taking the inverse:
g−1 ••y
A higher supergroup for string theory Introduction
Ordinary gauge theory describes how 0-dimensional particles transform as we move them along 1-dimensional paths. It is natural to assign a Lie group element to each path:
g • % •
since composition of paths then corresponds to multiplication:
g g0 • % • % • A higher supergroup for string theory Introduction
Ordinary gauge theory describes how 0-dimensional particles transform as we move them along 1-dimensional paths. It is natural to assign a Lie group element to each path:
g • % •
since composition of paths then corresponds to multiplication:
g g0 • % • % •
while reversing the direction corresponds to taking the inverse:
g−1 ••y A higher supergroup for string theory Introduction
The associative law makes the holonomy along a triple composite unambiguous:
g g0 g00 • % • % • % •
So: the topology dictates the algebra! For this we must ‘categorify’ the notion of a group!
A ‘2-group’ has objects:
g • % •
but also morphisms:
g % • f 9 • g0
A higher supergroup for string theory Introduction
Higher gauge theory describes the parallel transport not only of point particles, but also 1-dimensional strings. A ‘2-group’ has objects:
g • % •
but also morphisms:
g % • f 9 • g0
A higher supergroup for string theory Introduction
Higher gauge theory describes the parallel transport not only of point particles, but also 1-dimensional strings.
For this we must ‘categorify’ the notion of a group! but also morphisms:
g % • f 9 • g0
A higher supergroup for string theory Introduction
Higher gauge theory describes the parallel transport not only of point particles, but also 1-dimensional strings.
For this we must ‘categorify’ the notion of a group!
A ‘2-group’ has objects:
g • % • A higher supergroup for string theory Introduction
Higher gauge theory describes the parallel transport not only of point particles, but also 1-dimensional strings.
For this we must ‘categorify’ the notion of a group!
A ‘2-group’ has objects:
g • % •
but also morphisms:
g % • f 9 • g0 multiply morphisms:
g1 g2 % % • f1 9 • f2 9 • 0 0 g1 g2 and also compose morphisms: g
f g0 • / • C f 0 g00
A higher supergroup for string theory Introduction
We can multiply objects:
g g0 • % • % • and also compose morphisms: g
f g0 • / • C f 0 g00
A higher supergroup for string theory Introduction
We can multiply objects:
g g0 • % • % • multiply morphisms:
g1 g2 % % • f1 9 • f2 9 • 0 0 g1 g2 A higher supergroup for string theory Introduction
We can multiply objects:
g g0 • % • % • multiply morphisms:
g1 g2 % % • f1 9 • f2 9 • 0 0 g1 g2 and also compose morphisms: g
f g0 • / • C f 0 g00 again, the topology dictates the algebra.
Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse.
For higher gauge theory, we really want ‘Lie 2-groups’.
To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’.
But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’.
A higher supergroup for string theory Introduction
Various laws should hold... Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse.
For higher gauge theory, we really want ‘Lie 2-groups’.
To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’.
But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’.
A higher supergroup for string theory Introduction
Various laws should hold... again, the topology dictates the algebra. For higher gauge theory, we really want ‘Lie 2-groups’.
To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’.
But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’.
A higher supergroup for string theory Introduction
Various laws should hold... again, the topology dictates the algebra.
Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse. To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’.
But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’.
A higher supergroup for string theory Introduction
Various laws should hold... again, the topology dictates the algebra.
Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse.
For higher gauge theory, we really want ‘Lie 2-groups’. But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’.
A higher supergroup for string theory Introduction
Various laws should hold... again, the topology dictates the algebra.
Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse.
For higher gauge theory, we really want ‘Lie 2-groups’.
To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’. A higher supergroup for string theory Introduction
Various laws should hold... again, the topology dictates the algebra.
Just as a group is a monoid where every element has an inverse, a 2-group is a monoidal category where every object and every morphism has an inverse.
For higher gauge theory, we really want ‘Lie 2-groups’.
To study superstrings using higher gauge theory, we really want ‘Lie 2-supergroups’.
But to get our hands on these, it’s easiest to start with ‘Lie 2-superalgebras’. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
Let’s really start with ‘Lie 2-algebras’.
Roughly, this is a categorified Lie algebra: a category L equipped with a functor:
[−, −]: L × L → L,
where the Lie algebra axioms only hold up to isomorphism:
Axiom Lie algebra Lie 2-algebra Jacobi identity [x, [y, z]] + cyclic = 0 [x, [y, z]] + cyclic =∼ 0
(We’ll not weaken antisymmetry [x, y] = −[y, x]: this is called a semistrict Lie 2-algebra.) A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
A Lie 2-algebra contains a lot of information: objects, morphisms, and the isomorphisms weakening the Lie algebra axioms.
Fortunately, we can distill a Lie 2-algebra down to only four pieces of data: Theorem (Baez–Crans) Up to equivalence, every semistrict Lie 2-algebra is determined by the quadruple (g, h, ρ, J), where:
I g is a Lie algebra,
I h is a vector space,
I ρ: g → gl(h) is a representation of g on h, 3 I J :Λ g → h is a 3-cocycle in Lie algebra cohomology. In short: give me a Lie algebra 3-cocycle, and I’ll give you a Lie 2-algebra, and vice versa. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
‘3-cocycle’ in the sense of Lie algebra cohomology, which is defined using the complex which consists of antisymmetric p-linear maps at level p:
Cp(g, h) = {ω :Λpg → h}
We call ω a Lie algebra p-cochain.
This complex has coboundary map d : Cp(g, h) → Cp+1(g, h) defined by a long formula.
When dω = 0, we say ω is a Lie algebra p-cocycle. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
Remember, we really want Lie 2-superalgebras:
I g is a Lie superalgebra:
I a super vector space:
g = g0 ⊕ g1,
I with a graded-antisymmetric bracket [−, −], satisying the Jacobi identity up to some signs.
I h is a super vector space: h = h0 ⊕ h1.
I ρ: g → gl(h) is a homomorphism of Lie superalgebras. 3 I J :Λ g → h is a Lie superalgebra 3-cocycle. In short: give me a Lie superalgebra 3-cocycle, and I’ll give you a Lie 2-superalgebra. I For the right choice of S, there is a symmetric map:
[−, −]: Sym2S → V .
I There is thus a Lie superalgebra siso(n − 1, 1) where:
siso(n − 1, 1)0 = so(n − 1, 1) n V , siso(n − 1, 1)1 = S.
A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
The Poincaré superalgebra: n−1,1 I V = R has a nondegerate bilinear form g. I Spinor representations of so(n − 1, 1) are representations TV arising from left-modules of Cliff(V ) = vw+wv=2g(v,w) , since
so(n − 1, 1) ,→ Cliff(V ).
I Let S be such a representation. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
The Poincaré superalgebra: n−1,1 I V = R has a nondegerate bilinear form g. I Spinor representations of so(n − 1, 1) are representations TV arising from left-modules of Cliff(V ) = vw+wv=2g(v,w) , since
so(n − 1, 1) ,→ Cliff(V ).
I Let S be such a representation.
I For the right choice of S, there is a symmetric map:
[−, −]: Sym2S → V .
I There is thus a Lie superalgebra siso(n − 1, 1) where:
siso(n − 1, 1)0 = so(n − 1, 1) n V , siso(n − 1, 1)1 = S. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
Theorem For spacetimes of dimension n = 3, 4, 6 and 10, there is a Lie 2-superalgebra superstring(n − 1, 1) defined by the quadruple (g, h, ρ, J) where:
I g = siso(n − 1, 1) is the Poincaré superalgebra—the infinitesimal symmetries of ‘superspacetime’.
I h = R, I the action ρ is trivial,
I the 3-cocycle J is zero except for:
J(v, ψ, φ) = g(v, [ψ, φ]),
for a translation v and two ‘supertranslations’ ψ and φ. With some work, we can integrate superstring(n − 1, 1) to a ‘2-group’.
A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
Why superstring(n − 1, 1)?
I The classical superstring only makes sense in dimensions n = 3, 4, 6 and 10.
I In the physics literature, we see this is because J is a cocycle only in these dimensions. We can explain this using division algebras.
I Sati–Schreiber–Stasheff have a theory of connections valued in Lie 2-algebras. With superstring(n − 1, 1), the background fields look right. A higher supergroup for string theory Lie 2-superalgebras from 3-cocycles
Why superstring(n − 1, 1)?
I The classical superstring only makes sense in dimensions n = 3, 4, 6 and 10.
I In the physics literature, we see this is because J is a cocycle only in these dimensions. We can explain this using division algebras.
I Sati–Schreiber–Stasheff have a theory of connections valued in Lie 2-algebras. With superstring(n − 1, 1), the background fields look right. With some work, we can integrate superstring(n − 1, 1) to a ‘2-group’. A higher supergroup for string theory 2-groups from 3-cocycles
A 2-group is a category G with invertible morphisms, equipped with a functor called multiplication:
m : G × G → G.
The presence of isomorphisms allow us to weaken the group axioms:
Axiom Group 2-group Associativity (xy)z = x(yz) (xy)z =∼ x(yz) Left and right units 1x = x = x1 1x =∼ x =∼ x1 Inverses xx−1 = 1 = x−1x xx−1 =∼ 1 =∼ x−1x
These isomorphisms then must satisfy some equations of their own. In short: give me a group 3-cocycle, and I’ll give you a 2-group, and vice versa.
A higher supergroup for string theory 2-groups from 3-cocycles
The analogue of Baez and Crans’ theorem holds: Theorem (Joyal–Street) Up to equivalence, every 2-group is determined by the quadruple (G, H, α, a), where:
I G is a group,
I H is an abelian group,
I α: G → Aut(H) gives an action of G on H, 4 I a: G → H is a 3-cocycle in group cohomology. A higher supergroup for string theory 2-groups from 3-cocycles
The analogue of Baez and Crans’ theorem holds: Theorem (Joyal–Street) Up to equivalence, every 2-group is determined by the quadruple (G, H, α, a), where:
I G is a group,
I H is an abelian group,
I α: G → Aut(H) gives an action of G on H, 4 I a: G → H is a 3-cocycle in group cohomology. In short: give me a group 3-cocycle, and I’ll give you a 2-group, and vice versa. A higher supergroup for string theory 2-groups from 3-cocycles
‘3-cocycle’ in the sense of group cohomology, which is defined using the complex of G-equivariant maps:
p n p+1 o C (G, H) = F : G → H | gF(g0,..., gp) = F(gg0,..., ggp)
We call F a group p-cochain. This complex has coboundary map d : Cp(G, H) → Cp+1(G, H) defined by:
p+1 X i dF(g0,..., gp, gp+1) = (−1) F(g0,..., gˆi ,..., gp, gp+1). i=0 When dF = 0, we say F is a group p-cocycle. I But not vice versa: I haven’t defined Lie 2-(super)groups in general here, I am just using 3-cocycles as a substitute.
A higher supergroup for string theory 2-groups from 3-cocycles
For physics, we really need ‘Lie 2-groups’:
I G and H are Lie groups, α and a are smooth maps.
For the physics of superstrings, we really need ‘Lie 2-supergroups’:
I G and H are ‘Lie supergroups’, α and a are ‘super smooth’. In short: 4 I Give me a smooth 3-cocycle a: G → H, and I’ll give you a Lie 2-group. 4 I Give me a super smooth 3-cocycle a: G → H, and I’ll give you a Lie 2-supergroup. A higher supergroup for string theory 2-groups from 3-cocycles
For physics, we really need ‘Lie 2-groups’:
I G and H are Lie groups, α and a are smooth maps.
For the physics of superstrings, we really need ‘Lie 2-supergroups’:
I G and H are ‘Lie supergroups’, α and a are ‘super smooth’. In short: 4 I Give me a smooth 3-cocycle a: G → H, and I’ll give you a Lie 2-group. 4 I Give me a super smooth 3-cocycle a: G → H, and I’ll give you a Lie 2-supergroup.
I But not vice versa: I haven’t defined Lie 2-(super)groups in general here, I am just using 3-cocycles as a substitute. A higher supergroup for string theory Integrating Lie 2-algebras
So we have a Lie 2-superalgebra, superstring(n − 1, 1). But we want a 2-supergroup. It’s easy to go the other way:
I g is the Lie superalgebra of G,
I h is the Lie superalgebra of H,
I dα: g → gl(h) gives the induced action of g on h, 3 4 I Da:Λ g → h comes from differentiating a: G → H at 1, and antisymmetrizing. Da is still a cocycle. In general, there is a cochain map:
D : Cp(G, H) → Cp(g, h). A higher supergroup for string theory Integrating Lie 2-algebras
We want to solve the inverse problem. Our 3-cocycle formalism suggests a way to integrate Lie 2-superalgebras to Lie 2-supergroups:
I integrate g to G,
I integrate h to H,
I integrate the action ρ to an action α of G on H, 4 I find a cocycle a: G → H which somehow integrates J :Λ3g → h. More precisely, we want a cochain map:
∫ : Cp(g, h) → Cp(G, H)
which is the inverse of differentiation, at least up to chain homotopy: D : Cp(G, H) → Cp(g, h). A higher supergroup for string theory Integrating Lie 2-algebras
It’s not always possible to integrate cocycles, but it is for the defining cocycle J on the superstring(n − 1, 1) Lie 2-superalgebra.
This is because J is supported on the Lie subalgebra T = V ⊕ S, a nilpotent Lie superalgebra of translations and supertranslations. A higher supergroup for string theory Integrating Lie 2-algebras
As a warm up, we show how to integrate R-valued cocycles on any nilpotent Lie algebra n to cocycles on the corresponding group N, using a technique due to Houard.
In this case exp: n → N is a diffeomorphism, and gives a notion of straight lines in N. A higher supergroup for string theory Integrating Lie 2-algebras
p I Lie algebra cochains ω :Λ n → R can be identified with left-invariant differential forms on N.
I We can define left-invariant simplices in N to be simplices: p [n0,..., np]: ∆ → N, with the property:
n[n0,..., np] = [nn0,..., nnp].
I We integrate to get Lie group cochains on N: Z ∫ ω(n0,..., np) = ω. [n0,...,np]
I This defines a cochain map by Stokes’ theorem! A higher supergroup for string theory Integrating Lie 2-superalgebras
Now, we move into the world of supermanifolds: A higher supergroup for string theory Integrating Lie 2-superalgebras
Using a beautiful trick, called the functor of points. Theorem (Balduzzi–Carmeli–Fioresi) There is a full and faithful functor:
h: SuperManifolds → Fun(GrassmannAlg, A0-Manifolds).
So: for any supermanifold M and Grassmann algebra A, we get a manifold h(M)(A) = MA, the A-points of M.
For any map f : M → N of supermanifolds, we get a smooth map fA : MA → NA, which defines a natural transformation. A higher supergroup for string theory Integrating Lie 2-superalgebras
The functor of points allows us to replace a supermanifold M with a family of ordinary manifolds MA, and maps f : M → N with a family of smooth maps fA : MA → NA.
The A-points of a super vector space V are:
VA = A0 ⊗ V0 ⊕ A1 ⊗ V1. A higher supergroup for string theory Integrating Lie 2-superalgebras
The A-points of T are TA = A0 ⊗ T0 ⊕ A1 ⊗ T1.
I T a nilpotent Lie superalgebra ⇒ TA a nilpotent Lie algebra.
I J a 3-cocycle on T ⇒ JA a 3-cocycle on TA.
I TA has a group structure, TA ⇒ T has a supergroup structure T .
So: we integrate to get ∫ JA and transfer this back to T , defining ∫ J on T . A higher supergroup for string theory Integrating Lie 2-superalgebras
Theorem ∫ J defines a Lie supergroup 3-cocycle on T , which extends to a Lie supergroup 3-cocycle on SISO(n − 1, 1). Corollary There is a Lie 2-supergroup Superstring(n − 1, 1) integrating superstring(n − 1, 1). I This talk is based on a paper:
Division algebras and supersymmetry III
coming soon to an n-Category Café near you!
A higher supergroup for string theory Integrating Lie 2-superalgebras
Final thoughts:
I We want to do ‘higher gauge theory’ with Superstring(n − 1, 1). This should be related to string theory, and to the work of Sati–Schreiber–Stasheff.
I There is also a Lie 3-supergroup 2-Brane(n, 1) associated with super-2-branes. The higher gauge theory should be related to M-theory. A higher supergroup for string theory Integrating Lie 2-superalgebras
Final thoughts:
I We want to do ‘higher gauge theory’ with Superstring(n − 1, 1). This should be related to string theory, and to the work of Sati–Schreiber–Stasheff.
I There is also a Lie 3-supergroup 2-Brane(n, 1) associated with super-2-branes. The higher gauge theory should be related to M-theory.
I This talk is based on a paper:
Division algebras and supersymmetry III
coming soon to an n-Category Café near you!